textgraph v.1.1.0

• added functionality to save and load textgraph topic objects
• added functionality to visualize rwr terms
• added functionality for explore_topics(): open result without saving it; print top terms as table; different output formats
• added the option to calculate document relevance in calculate_dynamic_topics() and calculate_topics() as page rank in a document-document graph
• performance increases in calculate_dynamic_topics() and calculate_topics() via data.table implementation
• bug fixes in calculate_dynamic_topics(), calculate_topics(), classify_documents(), explore_topics()
• fixed erroneous widyr version requirement